(1) Field of the Invention
This invention relates to a process for producing clay derivatives having a porous structure and more particularly, to a process for producing stable composite materials which have large interlaminar spaces and are in the form of fine particles composed of clay minerals and hydrous metal oxides. The invention also relates to novel clay derivatives.
(2) Description of the Invention
Smectites, typical of which are montmorillonites, are a group of minerals constituting clay and have an interlaminar structure, in which exchangeable cations such as sodium, potassium, calcium, magnesium and the like are present in-between the laminar layers. Accordingly, it is known that various organic or inorganic ions or polar molecules are introduced into interlaminar interstices. It is usual that water molecules in the air are absorbed in the interstices of smectite. The space between the interstices is from approximately 3.0 to 5.8 angstrom and may be hereinafter referred to as "interlaminar interstices". However, the interlaminar interstices formed by the adsorption of moisture cannot be present stably because the moisture is readily desorbed on heating or under vacuum.
On the other hand, an attempt has been made to obtain a stable porous body by inserting bulky polynuclear metal ions into the interstices. More particularly, this is a process in which polynulcear ions or metal ions soluble in water are hydrolyzed with an alkali so that the resulting polynuclear hydroxide ions are introduced into interlaminar interstices of smectite, followed by thermal decomposition, thereby forming columns of the resultant oxide in the interstices. In the known process, polynuclear metal ions such as Al, Cr, Zr, Fe and the like are used to obtain a porous body having interlaminar interstices of 5 to 8 angstroms in width and a specific surface area of 200 to 400 M.sup.2 / g.
However, this porous body is disadvantageous in that when it is used as an encapsulating agent, adsorbent, catalyst, or carrier, it is required that an organic matter of a size depending on the purpose be adsorbed in a given amount, but the interstices of from 5 to 8 angstroms attained by the known process are not satisfactory. In addition, with the known porous body, it was difficult to control the size and adsorption of an organic matter to be adsorbed. The known process is also disadvantageous in that limitation is placed on the type of metal used. For instance, it was not possible to cause titanium oxide, which is versatilely used as a catalyst and particularly as a photo-catalyst in view of the electronic properties thereof or as a pigment because of the high refractive index, or copper or iron to be contained in the interstices. With regard to iron, there has been proposed a method in which a trinuclear iron acetate ion is used to obtain a porous body (Japanese Laid-open Patent Application No. 58-55332). This method is not generally applicable to other various metals.